I have a polite spine success story!
I have a friend, and he is lovely, but he can be a bit clueless. One of his worst habits is that he can rarely accept a no on the first go. He'll invite me out, and if I can't make it, I'll let him know, then I'll get another message going "Oh go on" and "Just one drink" and so on, and it drives me mad.
A few days back, I was having a really bad, stressful day, and in order to vent, I posted on a private group on FB a couple of times, but aforementioned friend was out all day so he didn't see it. He then invited me out to have drinks with him and a few other of our friends... Except he'd already asked me the day before, and I'd said no as I was still recovering from a cold. I gave him the benefit of the doubt and said no again. Sure enough, a few minutes later my phone buzzes, and it's him asking yet again. I got slightly short with him (he claimed we were "bartering", to which I replied "No we're not. Stop.") and I stopped replying to him.
So yesterday, I get a text from him, saying that he'd only just seen my posts on FB about what a bad day I'd been having, and he realised he'd been overly pushy on top of that. I sent back the following after rewriting it about a hundred times:
"Yeah, well please keep that in mind next time you ask me the same question 3-4 times and then ignore my response because it's not the one you wanted. You actually put me out quite a bit yesterday by doing that. I said no, as I have every right to. It was not an invitation to "barter" as you put it, and if you ever put it like that again you risk us falling out. But thank you for apologising."
He's since apologised again and promised that next time I say no, that'll be the end of the discussion. We'll have to see if it sticks, but he seems sincere about it.